In an image capturing apparatus such as a digital still camera or a camera recorder, when a shutter is pressed, a still picture is captured. For the captured original image data, internal signal processes such as a captured signal process and an encoding process are performed and then recorded for example to a detachable record medium. At this point, before the internal signal processes are performed for the captured original image data, they are temporarily stored in an image memory. The image memory is composed of a DRAM (Dynamic Random Access Memory), an SDRAM (Synchronous DRAM), or the like. As the number of pixels has increased in recent years, an image memory having a large storage capacity has become necessary. Thus, hardware cost and power consumption have adversely increased.
Thus, as described in Patent Document 1 (Japanese Patent Application Laid-Open No. 2002-111989), in an image capturing apparatus of the related art, captured original image data are compressed and stored in an image memory (an original image data buffer memory). Data that have been read from the image memory are decompressed and then the decompressed data are processed as described above. Compressing processes for original image data include DPCM process, Huffman encoding, arithmetic encoding for an original image signal, JPEG lossless encoding process that properly utilizes these processes, and universal encoding typified by Ziv-Lempel method.
In the DPCM process, the higher the correlativity of a pixel of interest and adjacent pixels becomes, the higher the compression ratio becomes. Thus, to obtain a high compression ratio, the DPCM process is effective for high order bits that have high correlativity, for example sixth to eighth bits of image data of 12 bits of one pixel. However, to do that, the DPCM process is not effective for low order bits that have low correlativity. In the DPCM, if an image that has low correlativity such as an edge of an image is compressed, the compressed image largely distorts. In the DPCM, an error propagates.
Compressed data that have been read from the image memory are decompressed and thereby original image data are obtained. For the original image data, signal processes such as Gamma correction, white balance correction, and linear matrix are performed. As a result, a luminance signal and two color difference signals are generated. The gamma correction is a process of which the camera side reversely corrects nonlinearity of light emitting characteristics of a Brown tube. In the gamma correction, the ratio of which image data having high luminance is compressed is high. Thus, according to characteristics of the gamma correction, a compressing system that uses nonlinear transform of which the ratio of which image data having high luminance is compressed is high has been proposed.
A compressing process that uses such nonlinear transform can be very easily structured. However, since the higher the compression ratio is, the more the information of low order bits is lost. As a result, image quality deteriorates as in solarization. Thus, it is difficult to obtain a high compression ratio. Solarization means a decrease of a developable concentration of a photographic emulsion because of excessive exposure. Visually, an image having a small number of quantizing bits, namely a rough gradation, is generated. As a result, the image quality of a flat image remarkably deteriorates.
Thus, in the compressing processes of the related art, compression distortions differ in their types. In any compressing process, there is a tendency that the higher the compression ratio is, the more the image quality deteriorates.
Thus, an object of the present invention is to provide an image data processing apparatus, an image data processing method, and a program that allow encoding efficiency to be improved so as to improve the image quality.